Heterobifunctional linkers possessing a reactive ester are useful for conjugating cytotoxic agents to a cell binding agent bearing an amine moiety, such as an antibody. Heterobifunctional linkers such as SPP and SPDB are typically prepared from monocarboxylic acid precursors by
esterification, an example of which is shown below:
If the monocarboxylic acids (I) were contaminated with dicarboxylic acid (III), then undesired homo-bifunctional linker (IV) would also be prepared as an impurity (see the reaction schemes above). It is known that di-carboxylic acid (III) can be generated as an impurity during the reaction of reactive disulfides with thiol-acid precursor (V) (see the reaction scheme below, U.S. Pat. No. 4,149,003). Therefore, purifications methods suitable for large scale commercial use for the separation of the desired monocarboxylic acid (I) from the undesired dicarboxylic acid (III) are needed.
